1. Field of the Invention
The invention pertains to the field of real image display systems. More particularly, the invention pertains to a real image display system incorporating two curved mirrors, at least one of which has an aspheric surface of revolution, or a tilted spherical mirror whose optical path does not reflect off of a beamsplitter surface.
2. Description of Related Art
It is desirable in modem imaging display systems to provide images having good contrast that appear sharp and undistorted to the viewer. One of the major problems in presenting a clear image is that real and infinity imaging systems generally employ curved mirrors and exhibit problems with secondary ghosting, that is, forming an additional image of the projected light source image at the point of observation. This occurs when outside light enters an imaging system and is projected as an additional ghost image near the focal point at which the primary image is projected and viewed.
The preferred solution to this problem is the use of a circular polarizer. The problem with this, however, is that conventional real image projection devices incorporate a beamsplitter tilted at 45 degrees to divert the converging imaging beam to form a real image outside of the device in viewer space. The circular polarizer will not block the ghosts when positioned at the window aperture of the system, because circularly polarized light will be distorted to elliptically polarized light when transmitting through a tilted beamsplitter, and will not transform back to circular when passing back through the beamsplitter after reflecting off of the curved reflector. The elliptically polarized light, therefore will not revert to linear after passing through the quarter wave plate on the second pass, and therefore will not be blocked. The compromise has been to position the quarter wave element of the circular polarizer between the curved mirror and the beamsplitter to avoid elliptical distortion, so the light is circularly polarized after passing through the beamsplitter. The difficulty with this approach is that the size of the quarter wave plate must be nearly equal in size to the curved mirror, and large quarter wave plates are not readily accessible or affordable. The size of the material required becomes smaller as its position nears the real image projection focal point, because the beampath is converging to form the real image. This elliptical distortion is the reason that prior art systems specify that the quarter wave element of the circular polarizer must be located between the beamsplitter and the curved mirror.
Briefly stated, a real image display system includes a primary image source for projecting a primary image from the start of a primary light path to an end of the primary light path at which the primary image is viewable, two reflectors positioned in the primary light path between the primary image source and the projected real image, a circular polarizer for circularly polarizing a light beam positioned in the primary light path between the mirror and real image, whereby outside light entering the system is substantially blocked before exiting the system, thereby substantially eliminating ghost image formation caused by outside light sources.
The circular polarizing plate is extremely effective, particularly in systems that do not include a beamsplitter positioned at a 45 degree angle to the optical axis, as in prior art systems, such as those described in U.S. Pat. Nos. 5,585,946 and 5,305,124. The systems disclosed in these patents place the circular polarizer elements between the mirror and beamsplitter, because of the effect of elliptical polarization caused by circularly polarized light passing through a beamsplitter at an angle other than normal to the surface. The circular polarizer placed at the window aperture of a conventional real image projection device containing a 45 degree positioned beamsplitter will not effectively block the ghost imaging.